There are manygreat recipes for these little treats. Some others use a home-made chocolate truffle to provide the molten chocolate filling in the middle. This one is a little less labor-intensive, but much harder to get right. Essentially, you need to remove the cakes from the oven just when they have set around the edges, with the centers still gooey.
As far as baking shapes go, I use a set of cylindrical metal tubing. These can be easily replaced with Pyrex or ceramic ramekins. My preference stems from the fact that these cylinders allow easy removal of the entire cake, without the risk of breaking it. I got them from amazon, they are called "cake rings".
(for 4 cakes)
2 tbsp butter
8oz semi-sweet chocolate
0.5 cup sugar
3.5 tbsp corn starch
4 egg yolks
vanilla ice cream for serving
Chop the chocolate, add the butter and melt gradually in the microwave, stirring occasionally.
Mix the sugar with the starch in a separate bowl. Beat the eggs in a third. Add the eggs gradually to the molten chocolate mixture (make sure it's not too hot), then mix in the sugar and starch.
Distribute into the four cake rings. Allw to cool in fridge overnight. Bake at 400F for about 8-14 minutes. This will hugely depend on the shape you decide to use, as well as your oven.
When done, cut around the cake and slide a metal spatula under the cake. Press out onto a serving plate. Serve with vanilla ice cream and fresh raspberries.